Our knowldege of the central neuropathways controlling respiration is incomplete. In addition, information regarding which neurotransmitters are involved in these pathways is virtually non-existent. In employing a new approach which combines immunohistochemistry with fluorescent dye tracing techniques we found evidence for a neural connection between the raphe and phrenic motor nucleus. We also localized the presence of serotonin, TRH and Substance P at the phrenic nucleus. Electrical stimulation of the raphe or 1. v. administration of a serotonin agonist completely stopped normal respiration and abolished spontaneous phrenic nerve activity. We propose to combine intracellular and extracellular electrophysiological techniques with immunohistochemistry and dye tracing to confirm this raphe-phrenic nucleus pathway. Specifically, we will determine the importance of this pathway in normal breathing and how it is modulated by other inputs into the respiratory centers. Also, we will employ pharmacological agents to determine the role of serotonin, TRH and Substance P in this pathway. Finally, we will determine if there is co-existence of transmitters in these neurons. Knowledge of the central inputs into the phrenic nucleus is important in understanding the normal process of breathing, but is critical in understanding disorders of this system. Central defects of the respiratory system resulting in muscle fatigue may be associated with chronic obstructive respiratory diseases as well as Central Sleep Apnea and Sudden Infant Death Syndrome. Determining the neural projections and neurotransmitters involved in the control of respiration should provide new information that will increase our understanding of the neural defects that can contribute to these disease states, and may offer a basis for developing new therapies for correcting these respiratory disorders.